The present invention is directed to a feed forward control for the expansion valve of a heating, ventilating, air conditioning or refrigeration (HVAC/R) system. In the preferred embodiment of the invention as described herein, the HVAC/R system is a water chiller system. Although discussed in terms of water chillers, the invention is applicable to all HVAC and refrigeration systems having system disturbances which can be anticipated. Such system disturbances include compressor staging, changes in compressor capacity such as those caused by loading or unloading, physical changes regarding the various cooling media used by the system such as changes in evaporator or condenser water temperature, changes in condenser cooling capacity such as those caused by fan staging, changes in evaporator heat exchanger capacity, changes in setpoint, changes in cooling tower capacity, and changes resulting from building load variations.
In water chiller systems, water is chilled in an evaporator so as to provide a cooling medium for air conditioning use elsewhere. Water is cheap, safe and can easily be transported by piping to an air handler by a first water loop. The air handler exchanges heat between air and water so as to condition the air for use in a zone or building.
The evaporator in a water chiller system is controlling the temperature of the water by heat exchange with refrigerant. The refrigerant circulates throughout the chiller system by means of a refrigerant loop. In the refrigerant loop, the refrigerant leaves the evaporator and enters a compressor where the pressure of the refrigerant is increased so as to change its condensation point. The compressed refrigerant leaves the compressor and enters a condenser where it is condensed from a vapor to a liquid refrigerant by heat exchange with a cooling medium, typically a second water system. The liquid refrigerant is then returned, by means of an expansion device, to the evaporator to continue the cycle through the chiller system. Aspects of typical chiller systems are shown in U.S. Pat. No. 4,780,061 to Butterworth; U.S. Pat. No. 4,762,409 to Tischer; U.S. Pat. No. 4,730,995 to Dewhirst; U.S. Pat. No. 4,662,190 to Tischer and U.S. Pat. No. 5,201,648 to Lakowske. All of these patents are assigned to the assignee of the present invention and all of these patents are incorporated herein by reference.
The expansion device in the chiller system is an electronic expansion valve which modulates refrigerant flow through the expansion valve in response to refrigerant superheat as measured after the refrigerant leaves the compressor. Typical electronic expansion valves are shown in U.S. Pat. No. 5,083,745 to Tischer; U.S. Pat. No. 4,986,085 to Tischer; U.S. Pat. No. 4,928,494 to Glamm and U.S. Pat. No. 5,011,112 to Glamm. These patents are assigned to the assignee of the present invention and are hereby incorporated by reference.
Typically, the compressor capacity is modulated in response to the leaving water temperature of the evaporator. Various methods of compressor capacity control and chiller capacity control are shown in U.S. Pat. No. 5,027,608 to Rentmeester et al.; U.S. Pat. No. 5,203,685 to Anderson et al.; U.S. Pat. No. 5,211,026 to Linnert; U.S. Pat. No. 4,715,190 to Han et al. and U.S. Pat. No. 4,689,967 to Han et al. Each of these patents is assigned to the assignee of the present invention and is hereby incorporated by reference.
While these various methods of controlling the expansion device and compressor capacities provide efficient and economical controls, better controls are both possible and desirable. More specifically, conventional control of the expansion valve is accomplished by feeding back a signal representing the result of the expansion valve's actions, that result typically being measured superheat. Such a control strategy, while effective, is reactionary as opposed to anticipatory. Thus, the expansion valve is constantly reacting to system disturbances such as changes in compressor capacity.